Fiber weave skew assessment for printed circuit boards

ABSTRACT

In one embodiment, a method includes inspecting a fiber weave for use in a printed circuit board with an automated optical inspection tool and identifying a distance between fiber bundles in the fiber weave. The fiber weave comprises a plurality of the fiber bundles woven to form the fiber weave and a portion of the fiber bundles comprise markers and identifying a distance between the fiber bundles in the fiber weave comprises measuring a distance between the markers.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 16/868,383 entitled “FIBER WEAVE SKEW ASSESSMENT FOR PRINTED CIRCUITBOARDS”, filed May 6, 2020, which is a divisional of U.S. patentapplication Ser. No. 15/872,163 entitled “FIBER WEAVE SKEW ASSESSMENTFOR PRINTED CIRCUIT BOARDS”, filed Jan. 16, 2018, now U.S. Pat. No.10,684,123, issued Jun. 16, 2020, the contents of which are herebyincorporated by reference in their entireties for all purposes.

TECHNICAL FIELD

The present disclosure relates generally to printed circuit boards(PCBs), and more particularly, to assessment of fiber weave skew inPCBs.

BACKGROUND

Fiber weave skew is an increasingly important problem for high speedserial links running differential pairs. The problem arises when thetime delay of one line in a differential pair is different than theother line. A common source of line-to-line skew is from local variationin the dielectric constant the lines see due to the inhomogeneous natureof the fiber-resin composite system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view showing a differential pair routed over afiber-resin composite system.

FIG. 2 illustrates an example of a reference plane used to identify alocation of the differential pair relative to a fiber weave.

FIG. 3 is a cross-sectional view of the fiber-resin composite system ofFIG. 1 showing dimensions associated with the fiber weave.

FIG. 4 is a table showing an example of skew assessment output, inaccordance with one embodiment.

FIG. 5 is a flowchart illustrating a process for assessing fiber weaveskew, in accordance with one embodiment.

FIG. 6A illustrates fiber bundle markers for use in evaluating fiberweaves, in accordance with one embodiment.

FIG. 6B is a flowchart illustrating an overview of a process forinspecting a fiber weave for use in a printed circuit board, inaccordance with one embodiment.

FIG. 7 illustrates automated optical inspection of a trace relative tothe fiber bundle markers, in accordance with one embodiment.

FIG. 8A is a top view showing circuit board inspection windows, inaccordance with one embodiment.

FIG. 8B is a flowchart illustrating an overview of a process forinspecting a layer of a printed circuit board through the inspectionwindow, in accordance with one embodiment.

FIG. 9A is a top view showing layers of a circuit board with the fiberbundle markers, in accordance with one embodiment.

FIG. 9B is a cross-section view of a circuit board showing inspectionwindows for each of the layers of FIG. 9A.

FIG. 10 is a perspective showing the inspection window in an explodedlayer view.

FIG. 11 is a perspective of the inspection window showing tracesrelative to the fiber bundle marker.

FIG. 12 depicts an example of a network device useful in implementingembodiments described herein.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

In one embodiment, a method generally comprises receiving a layoutidentifying routing information for a plurality of differential pairtraces on a printed circuit board at a skew assessment module, receivingvalues for a plurality of skew parameters associated with fiber weaveskew, receiving variation parameters from a database comprising datacollected on fiber weave variation for one or more of the skewparameters, calculating a fiber wave skew estimate based on the skewparameters and variation parameters at the skew assessment module, anddetermining if the fiber weave skew estimate is within a specified skewallowance.

In another embodiment, a method generally comprises inspecting a fiberweave for use in a printed circuit board with an automated opticalinspection tool and identifying a distance between fiber bundles in thefiber weave. The fiber weave comprises a plurality of the fiber bundleswoven to form the fiber weave and a portion of the fiber bundlescomprise markers. Identifying a distance between the fiber bundles inthe fiber weave comprises measuring a distance between the markers.

In yet another embodiment, a method generally comprises inspecting alayer of a printed circuit board through an inspection window comprisingan opening formed in one or more other layers of the printed circuitboard, and identifying a location of a trace aligned with the inspectionwindow, relative to a marker in a fiber bundle of a fiber weave toassess fiber weave skew.

Example Embodiments

The following description is presented to enable one of ordinary skillin the art to make and use the embodiments. Descriptions of specificembodiments and applications are provided only as examples, and variousmodifications will be readily apparent to those skilled in the art. Thegeneral principles described herein may be applied to other applicationswithout departing from the scope of the embodiments. Thus, theembodiments are not to be limited to those shown, but are to be accordedthe widest scope consistent with the principles and features describedherein. For purpose of clarity, details relating to technical materialthat is known in the technical fields related to the embodiments havenot been described in detail.

Typical PCB (printed circuit board) substrates are constructed fromvarious woven fiber fabrics strengthened and bound together with epoxyresin. As shown in the example of FIG. 1 , a PCB laminate is made up ofwoven fiber 10 impregnated with an epoxy resin 12. Due to thenon-homogenous nature of PCB laminates, the fiber weave based materialsmay cause signals to propagate at different speeds within differentialpair traces 14 a, 14 b. As shown in FIG. 1 , when one trace 14 a(positive trace P) lines up over a bundle of glass yarns 10 for aportion of its length, it will have a different propagation delay ascompared to another trace 14 b (negative trace N) of the same lengththat lines up over mostly resin 12. The two traces 14 a, 14 b shown inFIG. 1 have different effective dielectric constants (Dk) since resinand glass have two different dielectric constants. This results indifferent propagation velocities and may cause skew between P and Nsignals of the differential pair.

The effect described above is referred to herein as fiber weave skew(also referred to as glass weave skew, phase skew, timing skew,line-to-line skew, P/N skew, or skew). Skew can have a negative impacton s-parameters, eye margin (eye width, eye height, eye closure at areceiver), link performance, bit-error-rate (BER) performance, channelbudget, and Electro-Magnetic Interference (EMI), and becomes worse aslength increases. Fiber weave skew becomes more of an issue as bit ratescontinue to rise. For example, low skew is critical forserializer/deserializer (SerDes) speeds greater than 20 Gbps (high speedSerDes designs) (or other high speeds). The random alignment of trace tofiber bundles makes fiber weave skew difficult to diagnose.

The embodiments described herein provide for assessment of fiber weaveskew in PCBs. In one or more embodiments, relative skew of routed tracesmay be calculated to assess skew related PCB yield impact inmanufacturing and buy back eye margin lost due to glass weave skew onhigh speed signals. In one embodiment, a fiber weave skew assessmenttechnique estimates differential pair skew based on a number of designparameters and variation parameters. As described in detail below,output from the fiber weave skew assessment may provide a relative skewto identify traces that are likely to have more skew as compared toother traces in the PCB routing based on user defined variables and aknowledge database of known fiber weave variation. This may be used toenable designers to buy back performance improvements that wouldotherwise be lost due to fiber weave skew. Actual skew measurements onmanufactured boards may then be used to refine a PCB design. In one ormore embodiments, a fiber bundle marker may be used to screen weavestyles in manufacturing. Automated optical inspection may be used toperform layer analysis or inspect assembled boards through an inspectionwindow and provide skew related data based on location of a tracerelative to the bundle marker, as described further below.

PCB Design Fiber Weave Skew Assessment

In one or more embodiments, a fiber weave skew assessment technique maybe used to estimate differential pair skew on a routed PCB and therebyimprove yield in manufacturing. As described below, output from thefiber weave skew assessment is a relative skew unit used to identifytraces likely to have more skew relative to other traces for a PCBrouting based on user defined skew parameters and variation parameters.

In one embodiment, user defined skew parameters relating to routing,weave, and material are input to a fiber weave assessment algorithm. Theparameters may be defined to estimate fiber weave skew and may include,for example, one or more of the following parameters: differential pairrouting pitch; glass weave style from stack up; Dk data for glass fiberand resin in the stack up; glass weave bundle dimensions and resultingweave openings; short vs long grain panel to find warp direction/panelorientation; panel or PCB rotation degree; or any combination of theseor other user defined skew parameters.

The differential pair routing pitch is based on the routed design of thetrace. The term trace as used herein may refer to a chip-to-chip trace,one or more segments of a trace, or a differential pair trace. Thedifferential pair routing pitch may be defined by the routing of thedifferential pair on the circuit board as specified by a designer (ortrace routing program). This information may be obtained from animported PCB design layout. Additional design parameters such as panelor PCB rotation, panel orientation (short or long grain), and materialor weave selection may be user defined. Some of the parameters may bebased on the material (e.g., Dk data for glass fiber and resin), weavestyle (e.g., yarn type (E-glass, L-glass, NE-glass, etc.), weave type(e.g., as defined in IPC standard (e.g., 106, 1080, 2113, 2116, 3313,etc.)), spread type (based on manufacturer process), or glass (warp)direction (panel orientation) (short grain (e.g., 18 G×20), long grain(e.g., 18×20 G)).

FIG. 2 illustrates one example of a grid that may be used to define oneor more of the board layout geometry parameters. In one example, a meshgrid 20 is established at each layer with Δx_(i) and Δy_(i) defined asoffsets in x and y direction, respectively, where “i” refers to a layerat which the grid is defined. A variable θ° refers to an angle betweengrids at “i” layers to analyze angled routing. FIG. 2 shows routingtraces 14 a, 14 b relative to fiber weave 10 and dotted grid 20 providedas a reference.

In addition to the user defined skew parameters, variation/sweepparameters may be defined to control variation of one or more parameterssuch as bundle dimension and relative location of weave with respect toa differential pair. The IPC standard defines for different glass stylesand weave, a warp count, fill count, warp yarn, fill yarn, fabricthickness, and fabric nominal weight. However, the IPC standard does notdefine widths and gaps of glass bundles, which may vary based on weavestyle, material, manufacturing process, or manufacturer.

FIG. 3 is a cross-section of a composite sheet illustrating howvariation parameters may be defined in accordance with one embodiment.The composite sheet includes bundles 10 surrounded by epoxy resin 12.The bundles 10 are woven as shown in the top view of FIG. 1 , thus thebundles 10 are shown in cross-section in one direction and as acontinuous thread in a generally perpendicular direction. The fiberweave shown in the example of FIG. 3 is formed from glass bundlescomprising glass yarn. The bundle pitch/glass weave pitch is defined asthe distance between two adjacent and generally parallel bundlesmeasured from center to center and may be calculated from bundle countper inch. A bundle width is defined as the width of the bundle 10 withinthe resin 12. A gap is defined as the distance between two parallelbundles and a bundle thickness is defined as the thickness (height) ofthe bundle as shown in FIG. 3 .

The user defined skew parameters along with the variation parameters maybe modified during the assessment to control variation of some of theparameters such as bundle dimensions and relative location of weave withrespect to a differential pair. The fiber weave bundle dimensions andresulting weave openings, along with typical variations for thesemeasurements may be based on data collected in a knowledge database. Inone example, samples produced by different manufacturers, for differentmaterial types, glass weave styles, glass and resin material, spreadprocess, glass direction, or any combination of these or other factors,may be analyzed by various means and statistical data collected andstored in the knowledge database. Data may be collected for differenttypes of glass and resin, different spreading techniques (e.g., 1 sidevs. 2 side spreading), warp/fill data and hole size dimensions (holesize may be different in warp and fill directions), dimensions of glassbundles per glass style, or any combination of these or otherparameters. In one example, one or more parameters may be defined basedon the selected glass. The pitch dimension may be defined as a minimumand maximum bundle pitch dimension in prepeg and core configuration inwarp/fill directions. Variation parameters may also be defined for Dkdata for glass and resin and cross-section geometry for glass and traceconfigurations.

As described above, the parameters input to the fiber weave skewassessment include user defined skew parameters (e.g., routingparameters based on board layout, material selection, weave style, panelorientation (short/long, rotation), etc.) and variation parameters(e.g., weave geometry (e.g., minimum/maximum bundle dimensions) materialvariation, etc.) based on data collected for different configurationsamples from different manufacturers.

It is to be understood that the parameters described herein are onlyexamples and any combination of these or other parameters may be used toestimate skew for fiber weave skew assessment. As previously described,one or more parameters may be based on a knowledge database builtthrough measurement of manufacturing samples (e.g., test coupons) andused to fine tune one or more parameters. As described below, the designmay be adjusted based on an initial assessment and an iterative processmay be used to buy back margin from fiber weave skew. For example, userdefined skew parameters may be input for an asymmetry condition, asolution space defined with the variation parameters (e.g., fromknowledge database), and a relative skew defined as a function of datafrom the solution space. The skew analysis thus depends on user definedskew parameters and variation parameters that form a solution space usedin the fiber skew assessment process.

The assessment process may be performed at a fiber skew assessmentmodule in communication with the knowledge database and operable toreceive user input (e.g., board layout, material selections, weavestyle, etc.) through a user interface (e.g., graphical user interface(GUI), communication link, API (application programming interface) forimporting board design layout).

FIG. 4 is table illustrating an example of output data from a fiberweave skew assessment analyzer (module) for a fiber skew assessmentprocess. The output is for three traces (Trace 1, Trace 2, Trace 3),with each trace (e.g., differential pair) broken up into three segments(A, B, C) and a segment skew provided for each segment of the trace. Thesegment may comprise, for example, a horizontal portion, angled portion,and vertical portion as shown in FIGS. 1 and 2 . The total skew for eachtrace is a cumulative skew (e.g., skew A+skew B+skew C). The total skewis a relative skew unit that identifies the traces that are likely tohave more skew relative to other traces for the given PCB routing basedon the user defined solution space described above. The actual skew maybe solved for using a field solver algorithm, for example, however, thiswould use a large amount of resources (processing). The relative skewmay be used to determine if a trace is within a specified budget orlimit (e.g., <10s or any other specified limit).

FIG. 5 is a flowchart illustrating a process for assessing fiber weaveskew in a PCB design, in accordance with one embodiment. At step 50, aPCB design layout identifying routing information for a plurality ofdifferential pair traces on a printed circuit board is imported (input,received) at a skew assessment module (e.g., program, code, logic,computer, network device). Values for skew based parameters associatedwith fiber weave skew are also entered (step 51). The values may beinput, for example, at a graphical user interface or programmed for aselected design. As previously described, the skew parameters arespecific to a PCB. Common segments of the PCB may be broken up intounique topology segments for faster analysis (step 52). The layoutdesign and skew based parameters are used to calculate a relative skewfor each of the topology segments (step 53). The skew results for thesegments are cascaded after analysis to provide a skew estimate for achannel (e.g., chip-to-chip differential pair trace) (step 54).

If the skew (relative skew) is not within budget (step 55), the processmay be started over at step 50, with a designer varying one or more skewbased parameters (e.g., PCB design layout, glass weave, material, etc.).For example, a designer may vary skew parameters such as glass style,resulting in a different stack up, or PCB routing options such as angleor differential pair pitch to improve the skew on differential pairrouting. The skew budget may be a predefined or specified relative skewlimit per channel or for a layer of the board, or a complete PCB, forexample.

If the relative skew is within budget (step 55), the variationparameters may then be used to determine if the skew estimate for thechannel is still within acceptable limits (steps 56, 57, and 58). Forexample, if the skew estimate (relative skew) meets a specified skewbudget after initial simulation, variation/sweep parameters may beincluded to get an estimate of the minimum/maximum skew for the channel.As previously described, the variation parameters may be received from adatabase comprising data collected on fiber weave variation for one ormore of the skew parameters. A skew estimate is calculated for theprinted circuit board based on the skew parameters and the variationparameters and it is determined if the skew estimate is within aspecified skew allowance for the printed circuit board design (step 58).The skew allowance at step 58 may be per channel, layer, or for theentire PCB and may be the same value as used at step 55 or a differentlimit to account for additional offset due to variation parameters. Ifthe relative skew estimate is not within budget at step 58, the processmay be started over at step 50 with one or more new user defined skewparameters.

If the calculated skew is within budget at step 58, the PCB may bemanufactured and circuit board compliance may be verified using anyappropriate testing means, including for example, glass marker orinspection window techniques described below (step 59). If the skewanalysis of the manufactured PCB is within acceptable limits, the skewestimate process is complete (steps 59 and 60), otherwise the PCB may bescrapped (step 61) and the process started over at step 50 using theknowledge learned in the first iterative process or a new part may bemanufactured to see if the defect was due to a manufacturing error.

It is to be understood that the process shown in FIG. 5 is only anexample and that steps may be added, removed, combined, or modified,without departing from the scope of the embodiments. For example, thefiber skew analysis process as set forth in steps 50-58 may be performedon its own without a manufacturing verification (steps 59, 60, and 61).It should be noted that the process described above refers to one chipto chip channel broken down into segments, however, the process may alsobe performed for an aggregation of skew over layers. Also, themanufacturing compliance verification process (described below) may beperformed on its own, without the initial skew assessment process.

PCB Manufacturing Fiber Weave Skew Assessment

As described above with respect to the process of FIG. 5 , actual skewmeasurements (or relative skew measurements) may be used to refine thePCB design or determine if a manufactured board is in compliance with aspecified skew allowance. In one or more embodiments, a fiber bundlemarker may be used to screen fiber weave styles in manufacturing.Automated optical inspection may be used to perform layer analysis or aninspection window may be created in the PCB to provide a skewassessment, as described below.

In one or more embodiments, a glass marker image may be used to screendifferent glass styles and determine variation in bundles. Each glassstyle may have a different marker image based on its dimensions. In theexample shown in FIG. 6A, every fourth fiber (glass) bundle 62 comprisesa marker 65. The marker 65 may be inserted at any interval (with anynumber of bundles 10 therebetween) and may be inserted in bothdirections (warp and fill directions). For simplification, markers 65are only shown in one direction in FIG. 6A. The marker 65 may be used tocheck glass style quality as received from a manufacturer and also todefine variation parameters (e.g., collected data in knowledge database)for use in the fiber skew assessment analysis described above. Criteriafor screening may be user defined as a glass weave pitch±x mils as acheck at every N bundles, for example. Based on simulation data,accepted tolerances may be defined for scrapping criteria. The screeningcriteria may be dependent on cost or design, for example. As describedbelow, the markers 65 may also be used to help with orientation oftraces with respect to glass weave in warp and fill directions.

In one or more embodiments, the marker 65 may be embedded within theglass using any color strand or indicator. For example, when the glassis woven, some of the yarns may be a different color or have a visiblemarker embedded therein. Different color wavelengths may be used fordifferent glass styles. The marker may also be defined in the PCB designand etched on the PCB every N bundles in both warp and fill directions.The etched marker may be incorporated into the PCB design to perform analignment check and may be a copper trace placed at the same time thattraces are etched into the layer, for example.

FIG. 6B is a flowchart illustrating an overview of a process forinspecting a fiber weave, in accordance with one embodiment. In one ormore embodiments, the fiber weave is inspected with an automated opticalinspection (AOI) tool (step 67). A distance is identified between fiberbundles in the fiber weave (step 69). As described above with respect toFIG. 6A, a portion of the fiber bundles 62 comprise markers 65 (e.g.,marker embedded in glass bundle, etched marker). The distance betweenthe fiber bundles is identified by measuring a distance between themarkers 65. The measured distance may be a relative distance identifiedby the AOI or an actual measurement taken between markers.

As shown in FIG. 7 , the embedded marker 65 may be used to identify anorientation or offset of a trace 74. The alignment marker 65 may bealigned to the etched PCB trace 74 using an AOI tool (as indicated at76) with specific resolution to find anomalies.

The AOI tool allows for automated visual inspection of the fiber weave,layer (or layers) of a PCB, or a PCB through an inspection windowdescribed below. In one example, a camera or other optical measurementdevice may automatically scan the unit under test. The AOI tool may beconfigured to identify an offset between the traces and embedded markersor fiber weave skew (or relative skew) that exceeds allowable limits. Inone or more embodiments, an algorithm may be used to calculate fiberweave skew based on measured offsets.

The marker 65 may be applied, for example, every 10 bundles across thefiber (e.g., 17 mils wide with a marker every 10 bundles that covers 170mils+17 mils for the 11^(th) bundle, or at any other interval) to allowan AOI tool to accurately determine copper trace placement relative to aglass bundle center line, as well as calculate locational offset. AOImay be used to perform core layer analysis including bundle widthaccuracy, weave consistency across the panel, and copper trace offset toglass bundle center. This allows a core layer to be evaluated before itis assembled into the circuit board book for pressing.

In one embodiment, one or more inspection windows may be created toinspect one or more layers of a PCB after the PCB is assembled. FIG. 8Ais a top view of a circuit board showing inspection windows 84 locatedwithin a circuit board 82 and outer frame 83. The circuit board frame(also referred to as a circuit board panel) 83 extends around thecircuit board 82 and includes a flow dam area 80. The frame and circuitboard may include any number of bundle inspection windows 84 at multiplelocations for inspection of multiple layers within the board asdescribed below. The inspection windows 84 in the frame 83 allow forboard inspection without impacting the circuit board design.

FIG. 8B is a flowchart illustrating an overview of a process forinspecting a layer of the printed circuit board through an inspectionwindow, in accordance with one embodiment. A layer of the printedcircuit board is inspected through the inspection window comprising anopening formed in one or more other layers of the printed circuit board(step 87). As described below, multiple inspection windows may be formedto inspect multiple layers of the circuit board. A location of a trace74 aligned with the inspection window 84 is identified relative to amarker 65 in the fiber bundle of the fiber weave to assess fiber weaveskew (step 89) (FIGS. 7, 8A, and 8B).

FIG. 9A is a top panel view of multiple layers 90 (Layer 1, Layer 2,Layer N) comprising copper traces 94 and bundle markers 95. The term“layer” as used herein may refer to a copper sheet and fiber weave/resincomposite material, a core comprising a fiber weave/resin layerinterposed between two copper sheets, or any other combination of copperand fiber weave/resin layers. FIG. 9B is a side circuit board viewshowing the three stacked layers (Layer 1, Layer 2, Layer N) of FIG. 9Awith glass and copper punch-outs (openings) 98 a, 98 b, 98 c formed inthe layers to provide an inspection window for each layer. Opening 98 ais formed in Layer 2 and Layer N to create an inspection window alignedwith the trace 94 to view the location of the trace relative to thebundle markers 95 in Layer 1. Openings 98 b are formed in Layer 1 andLayer N to provide inspection windows above and below Layer 2. Openings98 c are formed in Layer 1 and Layer 2 to provide an inspection windowto view Layer N from above (as viewed in FIG. 9B). Window gaps 96 areprovided between the layers to prevent dry fill. The gap 96 providesenough resin to fill the void during a press cycle.

FIG. 10 is a perspective exploded view showing copper sheets (Cu 1, Cu2, Cu 3) and B-stages (fiber weave+resin) 105. In this example, openings108 are formed in copper sheets Cu 2 and Cu 3 and the B-stage interposedbetween these copper sheets to form an inspection window for viewingaligned copper traces 104 etched in copper sheet Cu 1 relative to thebundle markers 95 in the adjacent B-stage. All layers 102, 105 areassembled in order for the press cycle. In the example shown in FIG. 10, a square punch 108 (e.g., 200 mils by 200 mils or any other suitablesize or shape) is surrounded by a flow dam 110 in the copper sheet tocontrol resin depletion during the press cycle. The punch-out 108 may beformed during panel alignment hole punch. The flow dams 110 are providedto control resin flow during the press/thermal cycle to make and curethe circuit board. After the outer copper layers are etched, the window108 is created showing copper traces 104 relative to the marker bundles95 in weave 105 and resin is filled the rest of the way through. An AOI(e.g., microscope with measuring lines) may be used to determine thetrace location relative to the glass bundle.

It is to be understood that the layout shown in FIG. 10 is only anexample and any number of inspection windows may be formed foridentifying skew in any number of layers or PCB assembly comprising anynumber of cores, layers, copper sheets, or fiber weave+resin layers. Forexample, each layer may include any number of inspection windows 108 forviewing traces 104 relative to marker bundles 95 and determining fiberweave skew in one or more layers. The inspection may be performed on aplurality of layers (e.g., one or more core) or on the assembled circuitboard.

FIG. 11 is a perspective illustrating an inspection window 118 forviewing traces relative to a marker bundle 115. For simplification glassbundles are only shown in one direction. The layer includes differentialpairs 114 a, 114 b, and 114 c. A viewer or AOI tool may be used tomeasure the traces relative to the marker bundle 115. In the exampleshown in FIG. 11 , a first and third differential pair 114 a, 114 c haveminimum skew since both of the traces are located on a glass bundle 111,while a second differential pair 114 b has maximum skew since one traceis on the glass bundle 111, while the other trace is located on theresin between the glass bundles 111, 115.

Limits may be defined for the acceptable differential skew that thecopper layer may have. Measurements gathered during AOI through theinspection window may be used to determine if the amount of fiber weaveskew is acceptable. After the etch process of a core layer, the skew maybe unacceptable, in which case it can be scrapped with minimal impact tothe total board yield. After the board press and etch process, the skewacross all layers may be unacceptable; in this case the board can bescrapped before the drilling and finishing processes.

The printed circuit boards described herein may operate, for example, ina network device within a data communications network or any othercomputer or electronic device (e.g., desktop, laptop, handheld device,wireless device, tablet, gaming device, mobile device, phone, etc.). Thenetwork may include any number of network devices in communication viaany number of nodes (e.g., routers, switches, gateways, controllers,edge devices, access devices, aggregation devices, core nodes,intermediate nodes, or other network devices), which facilitate passageof data within the network.

FIG. 12 illustrates an example of a network device (computer) 120 thatmay implement the embodiments described herein. The network device maycomprise one or more PCBs designed using one or more embodimentsdescribed herein or may be used to perform a fiber weave skew assessmentas described above with respect to FIG. 5 , for example. In oneembodiment, the network device 120 is a programmable machine that may beimplemented in hardware, software, or any combination thereof. Thenetwork device 120 includes one or more processor 122, memory 124,network interface (port) 126, and fiber skew assessment module (e.g.,code, software, logic, device) 128.

Memory 124 may be a volatile memory or non-volatile storage, whichstores various applications, operating systems, modules, and data forexecution and use by the processor 122. The network device 120 mayinclude any number of memory components. Memory 124 may include aknowledge database 125 storing collected data on variation parameters aspreviously described.

Logic may be encoded in one or more tangible media for execution by theprocessor 122. For example, the processor 122 may execute codes storedin a computer-readable medium such as memory 124. The computer-readablemedium may be, for example, electronic (e.g., RAM (random accessmemory), ROM (read-only memory), EPROM (erasable programmable read-onlymemory)), magnetic, optical (e.g., CD, DVD), electromagnetic,semiconductor technology, or any other suitable medium. In one example,the computer-readable medium comprises a non-transitorycomputer-readable medium. The processor 122 may process data receivedfrom the interface 126 and may perform one or more of the steps shown inFIG. 5 . The network device 120 may include any number of processors122.

The network interface 126 may comprise any number of interfaces(linecards, ports) for receiving data or transmitting data to otherdevices. The network interface 126 may include, for example, an Ethernetinterface for connection to a computer or network.

It is to be understood that the network device 120 shown in FIG. 12 anddescribed above is only an example and that different configurations ofnetwork devices may be used. For example, the network device 120 mayfurther include any suitable combination of hardware, software,algorithms, processors, devices, components, or elements operable tofacilitate the capabilities described herein.

Although the method and apparatus have been described in accordance withthe embodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations made to the embodiments withoutdeparting from the scope of the invention. Accordingly, it is intendedthat all matter contained in the above description and shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. An apparatus comprising: a fiber weave for use ina printed circuit board, the fiber weave comprising a plurality of fiberbundles woven to form the fiber weave; a plurality of traces etched intothe fiber weave; and a plurality of markers located in a portion of theplurality of fiber bundles, wherein the plurality of markers aredifferent from the plurality of traces and are located at specifiedintervals with respect to the plurality of traces based on which arelative skew of the plurality of traces is assessed.
 2. The apparatusof claim 1, wherein the plurality of markers are located in theplurality of fiber bundles in a warp direction and a fill direction. 3.The apparatus of claim 1, wherein each of the plurality of markers isembedded within one of the plurality of fiber bundles, and wherein eachof the plurality of fiber bundles comprises a glass bundle.
 4. Theapparatus of claim 1, wherein each of the plurality of markers comprisesa colored strand.
 5. The apparatus of claim 1, wherein the plurality ofmarkers are etched with a specified number of bundles of the pluralityof fiber bundles therebetween.
 6. The apparatus of claim 1, wherein theplurality of markers are located at a specified distance from oneanother in the fiber weave.
 7. An apparatus comprising: a plurality offiber bundles woven to form a fiber weave; a plurality of traces etchedinto the fiber weave; and a plurality of markers, different from theplurality of traces, located at a specified interval with respect to theplurality of traces based on which a fiber weave skew is assessed. 8.The apparatus of claim 7, wherein the specified interval includes aspecified number of fiber bundles therebetween two adjacent markers ofthe plurality of markers.
 9. The apparatus of claim 7, wherein theplurality of markers are located in the plurality of fiber bundles in awarp direction and a fill direction.
 10. The apparatus of claim 7,wherein each of the plurality of fiber bundles comprises a glass bundle.11. The apparatus of claim 7, wherein each of the plurality of markerscomprises a colored strand.
 12. The apparatus of claim 7, wherein theplurality of markers are etched having a specified number of fiberbundles from the plurality of fiber bundles therebetween.
 13. Theapparatus of claim 7, wherein each of the plurality of markers isembedded within one of the plurality of fiber bundles.
 14. A methodcomprising: forming a fiber weave for use in a printed circuit board byweaving a plurality of fiber bundles; inserting, at a specified intervalinto a portion of the plurality of fiber bundles, a plurality ofmarkers; identifying locations of the plurality of markers in the fiberweave with an optical inspection tool; and measuring an offset betweenthe plurality of fiber bundles based on identifying the locations of theplurality of markers.
 15. The method of claim 14, wherein inserting theplurality of markers is performed such that a specified number of fiberbundles from the plurality of fiber bundles are between two adjacentmarkers of the plurality of markers.
 16. The method of claim 14, whereinthe plurality of markers are located in the plurality of fiber bundlesin a warp direction and a fill direction.
 17. The method of claim 14,wherein inserting the plurality of markers includes: embedding aplurality of colored strands into a glass bundle.
 18. The method ofclaim 14, wherein inserting the plurality of markers includes: etching acopper marker, which is one of the plurality of markers, into the fiberweave together with a routing trace being formed on the printed circuitboard.
 19. The apparatus of claim 1, wherein the plurality of markersare alignment markers based on which a skew of the fiber weave isassessed.